1. Field of the Invention
The invention relates to the removal of minerals, particularly silica, from water.
2. Description of the Prior Art
Removal of silica, a weakly ionized salt, from aqueous solutions as a final step in purification has heretofore been a difficult and costly process achieved principally by the use of strongly basic anion exchange resins in the hydroxyl (OH) form. The resin particles or beads are ordinarily arranged in a column configuration through which the solution is filtered. The resins require frequent regeneration as they become exhausted through use.
In an improved alternate approach, U.S. Pat. No. 3,149,061, Parsi, (which is expressly incorporated herein by reference;) discloses a method in which ion exchange resin particles are disposed in the fluid flow chambers formed by an arrangement of alternating anion and cation permeable membranes in an electrodialysis cell providing a combination of resin exchange and electrodialysis treatment. In addition to promoting the electrodialysis transfer of silica, the resin particles are self-regenerated by the action of the cell in splitting water into H.sup.+ and OH.sup.- and therefore does not require a separate regeneration step.
It has now been discovered that by adjusting the pH of the subject solution, silica removal may be carried out in a conventional electrodialysis cell without any use of resin particles; completely eliminating the need for such material and any requirement for specially designed electrodialysis cells containing the resin particles.
The most simple electrodialysis cell comprises a cathode at one terminal end, an anode at the opposite end, and at least two ion permeable membranes. One membrane is selectively cation permeable, and the other anion permeable, and together form a diluting (desalting) chamber therebetween. As is well known, commercial electrodialysis cells are most often comprised of many alternating pairs of cation and anion permeable membranes forming a plurality of salt diluting and salt concentrating chambers. Direct current is supplied between the anode and cathode. In the present invention it is preferable that the solution to be treated is substantially free of strongly ionized salts (such as NaCl) preferably less than 200 ppm. The substantially desalted solution still containing silica as silicic acid is then passed into the diluting chambers together with a sufficient quantity of a hydroxide, preferably NaOH, to raise the pH to at least about 9.5 but preferably not greater than 11.5. As described above, silica now in the anion form will migrate out of the feed (dilute) stream through the anion membrane and the sodium cation will migrate out of the feed stream through the cation membrane; the ions eventually ending up in the concentrating (waste) stream.
It has been found that too high a pH will result in decreased removal of silica due to competition from excess hydroxyl anions. An upper pH limit of about 11.5 has been discovered as the value at which the efficiency of the silica removal process begins to decline.
Although NaOH is the preferred agent for increasing the pH, other compounds such as NH.sub.4 OH and KOH have been found to promote transfer as well. Additionally, the efficiency of the process is increased it the anion membranes are first placed or equilibrated into the hydroxyl form prior to initiation of the process. This conditioning of the anion membrane into the hydroxyl form will naturally evolve through cell operation, however, so such a prior step is not essential to the process.